Conjugated polymers based on dicarboxylic imide-substituted isothianaphthene and their applications in solar cells

Four new polymers containing a benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic imide (DIITN) unit including the homopolymer and three donor–acceptor copolymers were designed, synthesized, and characterized. For these copolymers, DIITN unit with low bandgap was selected as an electron acceptor, whereas 5,5′-(2,7-bisthiophen-2-yl)-9-(2-decyltetradecyl)-9H-carbazole), 5,5′-(3,3′-di-n-octylsilylene-2,2′-bithiophene), and 5,5′-(2,7-bisthiophen-2-yl-9,9-bisoctyl-9H-fluoren-7-yl) were chosen as the electron donor units to tune the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) levels of the copolymers for better light harvesting. These polymers exhibit extended absorption in the visible and near-infrared range and are soluble in common organic solvents. The relative low lying HOMO of these polymers promises good air stability and high open-circuit voltage (Voc) for photovoltaic application. Bulk heterojunction solar cells were fabricated by blending the copolymers with [6,6]-phenyl-C61-butyric acid methyl ester or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The best power conversion efficiency of 1.6% was achieved under simulated sunlight AM 1.5G (100 mW/cm2) from solar cells containing 20 wt % of the fluorene copolymer poly[5,5′-(2,7-bisthiophen-2-yl-9,9-bisoctyl-9H-fluoren-7-yl)-alt-2,9-(benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic imide)] and 80 wt % of PC71BM with a high open-circuit voltage (Voc) of 0.84 V